Sillimaniet-houdende zwerfkeien

Contrary to the impression given by the available regional literature, sillimanite can be found in many ice or river transported
gneissic erratics in the Netherlands. Macroscopic and microscopic descriptions of two sillimanite-bearing erractic, collected
from different locations in the northeastern Netherlands are given (see fig. 2-10). A list and a map of occurrences of 17
sillimanite-bearing erratics known to the authors from finds in the Netherlands is added (see table I and fig. 11). The distribution
pattern of these particular erratics grossly correlates with the maximal extension of the Saalian ice sheet in this country.
As yet there are no reports of finds of such erratics from locations that have not been under direct or indirect influence
of the Saalian glaciation. Therefore, it is suggested that the sillimanite-bearing erratics listed have a Scandinavian provenance.
The conditions for the formation of sillimanite-bearing metamorphites in general, are reviewed. The unusual persistence of
patches of apparently non-devitrified glass in one of the high-grade gneisses described -a rock of deep-seated origin- is
tentatively explained as being due to their having a presumed abnormally alkali-rich and allumina-poor composition, preventing
the crystallization of a stable mineral paragenesis, even during slow cooling. It is suggested that droplets of anatectic
smelt, formed by ultra-metamorphism were unusually depleted in alumina by the crystallization of numerous sillimanite needles
that abound in the glass.